Attemping to add a value to a HashSet doesn't change the amount of values in it

I have a HashSet and when I use the Add method of the collection, nothing is added. The output is still 2, 3, 5, 7, 11, 13 and the output from .Count is 6.

Is this a bug or am I doing something wrong here?

namespace AllerDiz
{
    class MainClass
        {
            public static void Main (string[] args)
            {
                HashSet<int> smallPrimeNumbers = new HashSet<int> { 2, 3, 5, 7, 11, 13 };
                smallPrimeNumbers.Add (3);
                smallPrimeNumbers.Add (5);
                smallPrimeNumbers.Add (7);
                Console.WriteLine ("{0}", smallPrimeNumbers.Count);
                foreach(int val in smallPrimeNumbers)
                {
                    Console.WriteLine ("HashSet Value= {0}", val);
                }
            }
      }
}